Topographic development of the Southern Alps recorded by the isotopic composition of authigenic clay minerals, South Island, New Zealand

The Southern Alps are developing as a consequence of oblique collision between the Pacific and Australian plates. The Southern Alps lie on the west side of the South Island of New Zealand and create a massive rain shadow where greater than 12 m/year of rain falls on the west coast and semiarid conditions exist to the east. The rain-out effect across the mountains causes precipitation west of the Southern Alps to have delta D and delta(18)O values averaging -30 parts per thousand and -5.5 parts per thousand, whereas precipitation in the rain shadow to the east is isotopically lighter (delta D = -72 parts per thousand and delta(18)O = -9.8 parts per thousand). Such large differences in the isotopic composition of precipitation would not have existed prior to the development of significant topography. We have examined the topographic evolution of the Southern Alps using oxygen isotope analyses of authigenic kaolinites formed in the rain shadow to the east of the mountains between the Cretaceous (low topography) and the Pleistocene. Changes in the isotopic composition of authigenic clay minerals forming in equilibrium with meteoric water in the stratigraphic sequence record the development of Southern Alps topography and the resultant rain shadow. Our oxygen isotope analyses of authigenic kaolinites show a 5-6 parts per thousand decrease in the early Pliocene, from similar to 18.2 parts per thousand in older rocks, to similar to 12.3 parts per thousand in younger rocks. In addition, smectite is abundant in all samples from the Late Miocene to Recent, but is conspicuously absent in most older rocks, suggesting a change to a generally drier climate roughly coincident with the isotopic shift in kaolinites. This method may be useful in unraveling timing of development of mountain belts elsewhere in the world. (C) 1999 Elsevier Science B.V. All rights reserved.